High-temperature furnaces are used for a variety of industrial applications, however, troubles occur, such as the formation of scale or damage over a prolonged period of use, since the reaction products produced in the furnaces or dust are deposited therein. This necessitates coating the refractory surfaces of the furnaces with dense layers, or repairing the damaged portions by implantation of refractory materials.
When the furnaces are small, the furnaces may be stopped to take necessary measures without causing particular problems, whereas large furnaces which are often operated continuously for a long period usually undergo such measures while hot, without being cooled.
The methods of coating the refractory surfaces of furnaces while hot include spraying of monolithic refractories, plasma- or arc flame spraying, etc. The former method, though relatively inexpensive to perform, has drawbacks in that dense coating layers are difficult to form, and the coating layers are not sufficiently strong to withstand peel-off. On the other hand, the latter method has the economic drawback of high cost to perform, though relatively strong films may be formed.
A special example of the scaling mentioned above includes the problem of deposition of carbonaceous materials on the refractory surfaces of coke furnaces.
Coke furnaces produce coke by carbonization of coal at about 1,100.degree. C. for 20-25 hours. Tarry materials and hydrocarbon gases are produced during the process of carbonization of coal. Such products penetrate into cracks in the inner walls, furnace lids, coal inlets, etc. of coke furnaces or into the open gas-pockets of the refractories constructing the furnaces, and are thermally carbonized to form highly adhesive carbon deposits.
The carbon deposits lower the melting points of the refractories, and also cause the refractories to become brittle. In addition, the furnace lids become more difficult to open and close as the carbon deposits accumulate, resulting in poorer sealing of the coke furnaces with the furnace lids. Mechanical removal o f the deposited carbon, which is frequently carried out to prevent the problem, however, takes much time due to the strong adhesion, and further the work environment is harsh. In addition, the surfaces of the refractories themselves are sometimes removed away during the process of removal. An alternative method by which air or oxygen gas is blown to burn off the carbon, however, can be applied to only the neighborhood of the furnace inlets. In order to clean all areas of the furnaces, the furnaces must be stopped temporarily and emptied out to burn off the carbon. However, the burning-off itself is a rigorous high-temperature operation, and the combustion heat generated during the burn-off process creates local hot spots of the refractories of the furnaces which may damage the furnaces.
In view of the circumstances, a variety of studies have been carried out on carbon adhesion-proof refractories and methods of protecting the refractory surfaces by coating with films.
Representative examples include:
1) Japanese Examined Patent Application Publication SHO 62-19477: application of a composition comprising silicon carbide, silicon nitride or graphite particles and an inorganic binder to the lining bricks of coke furnaces;
2) Japanese Unexamined Patent Application Disclosure SHO 62-197371: application of an agent which imparts properties of resisting heat and preventing the penetration of tarry materials, and comprises silicon carbide, silicon nitride, etc., a binder comprising a phosphate, yttrium oxide, etc. and a thermal insulating property-imparting agent comprising potassium titanate fibers to the inner wall surfaces of coke furnaces;
3) Japanese Examined Patent Application Publication SHO 63-40463: application of graphite powder and an inorganic binder such as colloidal silica and alumina sol to the lining refractories for the doors of coke furnaces; and
4) Japanese Unexamined Patent Application Disclosure SHO 63-236783: concurrent firing of a glazing agent and bricks for preparation of refractory bricks with glazing layers formed thereon, for the construction of coke furnaces;
Of the methods mentioned above, Methods 1) through 3) which use silicon carbide, silicon nitride, graphite and so on suffer from poor compatibility between the particles of these materials and the binders, and this produces the problem of peel-off of the coating layers during operation of the furnaces due to insufficient bonding strength.
Method 4) which uses bricks with glazing layers formed thereon provides satisfactory adhesion which does not allow the glazing layers to peel off while the furnaces are in use. The method is very effective also in that few gas-pockets are present in the glazing coats, and thus no carbon penetrates. This method, though applicable to the manufacture of new coke furnaces, furnace lids, etc., cannot be applied to running furnaces while hot in order to form glazing layers on the refractory surfaces of the furnaces.
It is a first object of the invention, which has been accomplished in view of the problems mentioned above, to provide a method of hot-applying a glazing agent to the surfaces of bricks, joints and monolithic refractories which form the inner wall surfaces of high-temperature furnaces, to form glazing layers thereon by melting the glazing agent.
It is a second object of the invention to provide a method of applying a glazing agent to the inner wall surfaces of furnaces such as coke furnaces and melting the glaze to form dense, durable, high-performance coats on the refractory wall surfaces which prevent the deposition of carbon and facilitate peel-off of any carbon deposits to prolong the lives of the furnaces.